1. Field of the Invention
The present invention relates to a radar apparatus which is to be mounted on, for example, a vehicle, for switching from one of a plurality of transmission antennas to another to transmit a transmission signal as an electromagnetic wave, receiving the electromagnetic wave reflected from a target as a reception signal by a reception antenna, and detecting a distance to the target, a relative velocity to the target, and an angle to the target based on a reception signal.
2. Description of the Related Art
As one of the radar apparatus mounted on a vehicle and used in vehicle velocity/inter-vehicle control (such as adaptive cruise control (ACC)) or a collision mitigation brake system, there has been known a radar apparatus of a frequency modulated continuous wave (FMCW) system (hereinafter, referred to as “FMCW radar apparatus”), which can detect a distance to a target and a relative velocity to the target at the same time.
The FMCW system is one of the radar transmitting systems, and can calculate a distance to an object (target) and a relative velocity to the target by calculating a difference in frequency between a transmission wave and a reception wave (transmission wave reflected from the target).
The FMCW radar is obtained by subjecting a transmission signal of a continuous wave (CW) radar to FM modulation. That is, in the FMCW radar apparatus, a frequency of an oscillator is modulated by a triangular wave into a transmission signal which is radiated from the transmission antenna to the external. Further, the reception signal received by the reception antenna, which is the transmission signal reflected from the target, is subjected to a time lag caused by the distance to the target and a frequency shift corresponding to the relative velocity to the target.
Subsequently, the reception signal subjected to the frequency shift is mixed with the transmission signal, thereby obtaining a beat signal. Then, abeat frequency in an increasing chirp (up-chirp) where the frequency increases and a beat frequency in a decreasing chirp (down-chirp) where the frequency decreases are measured separately in each modulation cycle, thereby calculating the distance to the target and the relative velocity to the target. This technology is generally used in the FMCW radar apparatus, and is a known technology.
On the other hand, in order to detect an angle to the target, that is, an arrival angle of the reception wave, there has been known a method in which a plurality of antennas spatially arranged at different positions are used. As examples of such method, there have been known methods of Multiple Signal Classification (MUSIC) and Estimation of Signal Parameters via Rotational Invariance Techniques (ESPRIT).
In those angle calculating methods using a phase, it is known that more excellent angle resolution is generally obtained as the antenna aperture size is larger. However, when there is such a limit that the radar apparatus is located in a restricted space, the obtained angle resolution is limited. For that reason, a countermeasure is applied in which two transmission antennas are disposed on both ends of the plurality of reception antennas, an electromagnetic wave is transmitted in a time sharing manner while switching between those two transmission antennas, and an equivalent aperture about twice as large as the normal aperture is realized by using the respective reception results.
In the radar apparatus thus configured, when a transmission switch that switches between the two transmission antennas to select the transmission antenna for transmitting the electromagnetic wave has failed, and is fixed to an always-on or an always-off, the distance to the target and the relative velocity to the target can be normally calculated whereas the angle to the target cannot be normally calculated. That is, although an erroneous angle to the target is calculated, the calculation per se of the angle to the target can be performed. Therefore, it is difficult to detect a failure of the transmission switch.
There is known a radar apparatus which includes a plurality of transmission and reception antennas, and a plurality of transmission and reception switches that are disposed for the transmission and reception antennas so as to switch from one of the transmission and reception antennas to another, in which the transmission and reception switches are sequentially switched to compare levels of the reception signals of the respective transmission and reception antennas with each other, and the failure of the transmission and reception switches is detected based on the comparison results (see, for example, Japanese Patent Application Laid-open No. 2007-171162).
However, the related art suffers from the following problems.
That is, when the plurality of transmission and reception antennas and the plurality of transmission and reception switches are provided as in the radar apparatus disclosed in Japanese Patent Application Laid-open No. 2007-171162, all the combinations of transmission and reception of the transmission and reception antennas are executed to detect the failure of the transmission and reception switches.
On the other hand, in a radar apparatus having a plurality of transmission antennas and a plurality of transmission switches instead of the plurality of transmission and reception antennas and the plurality of transmission and reception switches, the transmission switches contribute to only the transmission of the electromagnetic wave, and the reception antennas are always in a reception state. Therefore, it is difficult to detect the failure of the transmission switches.
For example, consider a case where two transmission antennas (first transmission antenna and second transmission antenna) and two reception antennas are arranged, and a first transmission switch that selects the first transmission antenna is fixed to an on-state. In this case, an electromagnetic wave normally transmitted from the first transmission antenna is received by the two reception antennas in a transmission period of the first transmission antenna. However, in a transmission period of the second transmission antenna, electromagnetic waves transmitted from the first transmission antenna and the second transmission antenna at the same time are combined and received by the two reception antennas. In this case, it is difficult to detect the fixed on-state of the transmission switch by only the combination of switching of the transmission switches.